1. Field of the Invention
The present invention relates to a telescopic boom that is provided in a working vehicle, a crane and so on.
2. Description of the Related Art
There is known a telescopic boom in that the entire boom is telescopic, as a boom provided in a working vehicle, a crane and so on.
This telescopic boom is provided with a boom body that is telescopically formed in a longitudinal direction of the boom and in which a plurality of boom members are inserted in a telescopic manner from a base end of the boom body, and a telescopic cylinder that is positioned within a top boom member arranged in a tip end portion of the boom body at a completely contracted time of the boom body to extend and contract the boom body. A roller is rotatably provided at a tip end portion of the telescopic cylinder (for example, see Japanese Patent No. 3853149). The roller is in contact with a lower plate of the boom body and is configured to be capable of rolling on the lower plate in a longitudinal direction of the telescopic cylinder in accordance with the telescopic movement of the telescopic cylinder.
However, if the lower plate is formed to be thin in thickness for lightweight of the boom body, a bottom wall of the lower plate is deflected by a self-weight of the telescopic cylinder in the completely contracted and horizontal state of the boom body, and corners or peripheral edges of both ends of the roller collide with the lower plate.
Therefore, as the roller is inclined to the lower plate when a working vehicle vibrates at traveling, a peripheral surface of the roller makes point-contact with the lower plate to cause overconcentration of loads on the lower plate. In consequence, there is a possibility that a defect such as permanent deformation of the lower plate is brought about. When the deformation or the like is brought about in the lower plate, there is problem that the boom body cannot be smoothly extended or contracted.